Most overplayed = Champion. Without a doubt. In every group you'll end up with 2-3 and another 2-3 wanting to join. The reason is they are the strongest solo class in the game at the moment.
Most underplayed = Guardian or Burglar (with Lore-Keeper close behind)
Most useless class = There are no useless classes - they all fill different roles and some of them are flexible and can main a couple roles (ie., Rune keeper can dps or heal)
Most needed class = Guardian (THAT ACTUALLY KNOWS HOW TO CONTROL AGGRO) or Burglar. Burglar's early game is boring and you don't really see what they're capable of until you get to probably 30+ when they're ability to start Fellowship Manuevers and crowd control abilities really start to shine. Guardian, as with all pure tanks, is absolutely boring and slow to solo with but absolutely crucial and fun in groups. Lore-keepers ability to crowd control is absolutely priceless IF the lore-keeper understands how to crowd control AND the group understands how to LET them crowd control (ie., not touching dazed mobs). Unfortunately this doesn't tend to happen in pub groups before 50+ so it can be extremely annoying to play one.